The present invention relates generally to semiconductor packaging, and, more particularly, to a method of assembling a semiconductor pressure sensor device.
Semiconductor sensor devices, such as pressure sensors, are well known. Such devices use semiconductor pressure-sensing dies to sense the ambient atmospheric pressure. These dies are susceptible to mechanical damage during packaging and environmental damage when in use, and thus they must be carefully packaged. Further, pressure-sensing dies, such as piezo resistive transducers (PRTs) and parameterized layout cells (P-cells), do not allow full encapsulation because that would impede their functionality.
FIG. 1(A) shows a cross-sectional side view of a prior-art packaged semiconductor sensor device 100. FIG. 1(B) shows a perspective top view of the sensor device 100 partially assembled, and FIG. 1(C) shows a perspective top view of the sensor device 100 with a lid 104 attached.
As shown in FIG. 1, the sensor device 100 is encased in a molding compound 102, which has a cavity 124 formed therein. Inside the cavity 124, a pressure sensor die (P-cell) 106, an acceleration-sensing die (G-cell) 108, and a micro-control unit (MCU) die 110 are adhesively-attached onto a lead frame flag 112 (sometimes referred to as a die paddle). The P-cell 106, G-cell 108, and MCU die 110 are electrically interconnected to one another and to lead frame leads 118 via one or more bond wires 120 (one of which is shown in FIG. 1(A)).
The P-cell 106 is designed to sense ambient atmospheric pressure, while the G-cell 108 is designed to sense gravity or acceleration in one, two, or all three axes, depending on the particular implementation. The MCU 110 controls, for example, the operations of and the processing of signals generated by the P-cell 106 and the G-cell 108. The P-cell 106, the G-cell 108, and the MCU 110 are well-known components of semiconductor sensor devices and thus detailed descriptions thereof are not necessary for a complete understanding of the present invention.
The cavity 124 is partially filled with a pressure-sensitive gel material 114, which enables the pressure of the ambient atmosphere to reach the pressure-sensitive active region on the top side of the P-cell 106. The pressure-sensitive gel material 114 protects all of the dies 106, 108, 110 and the one or more bond wires 120 from mechanical damage during packaging and environmental damage (e.g., contamination and/or corrosion) when in use. The cavity 124 is covered by a lid 104, which has a vent hole 116 that exposes the gel-covered P-cell 106 to ambient atmospheric pressure outside the sensor device 100.
The gel material 114 is applied using, for example, a nozzle of a conventional dispensing machine (not shown), as is known in the art. After dispensing, the gel material 114 is cured by, for example, placing the sensor device 100 in an oven. Note, however, that there is commonly a delay between the time that the gel material 114 is dispensed and the time that the gel material 114 is oven-cured. For example, this delay can be caused by normal production flow delays such as waiting for space to become available in the oven.
As another example, if the sensor device 100 is assembled as part of a strip (i.e., a one row, multi-column array) or a magazine of strips (i.e., a multi-row, multi-column array) of similar sensor devices (not shown) on a shared substrate, then the gel material 114 may need to be dispensed onto one or more of the other similar sensor devices, after being dispensed onto sensor device 100. Thus, there is a delay while waiting for the gel material 114 to be dispensed onto the other similar sensor devices.
Once the gel material 114 has been dispensed onto all of the sensor devices, the gel material 114 for all of the sensor devices on the strip or magazine can be cured concurrently by placing the whole strip or magazine into the oven. After curing, the sensor devices on the strip or magazine can be separated from one another using, for example, saw or laser singulation.
During the delay between dispensing and oven-curing, capillary action shapes the upper surface 122 of the gel material 114. The capillary action occurs because the adhesion of the gel material 114 to the inner walls of the molding compound 102 is stronger than the cohesive forces between the molecules of the gel material 114. The adhesion causes an upward force on the gel material 114 at the inner walls of the molding compound 102 and results in a meniscus (i.e., the upper surface 122) that turns upward. The height to which the gel material 114 climbs the inner walls of the molding compound 102 is limited by surface tension and gravity.
If the curvature of the upper surface 122 becomes too great, then portions of the one or more bond wires 120 (and possibly even portions of the dies 106, 108, and 110) may become exposed to the ambient atmosphere above the gel material 114 as shown in FIG. 1(A). Exposure of the one or more bond wires 102 and the dies 106, 108, and 110 to the ambient atmosphere can result in environmental damage (e.g., contamination and/or corrosion) to the sensor device 100.
To prevent portions of the bond wires 102 and dies 106, 108, and 110 from becoming exposed, the amount of the gel material 114 dispensed by the dispensing machine for each sensor device could be increased. However, increasing the amount of the gel material 114 dispensed could have several adverse effects.
First, increasing the amount of the gel material 114 dispensed increases the cost of each sensor device, and possibly, the size of each sensor device if the cavity needs to be made deeper to prevent the gel material from coming too close to the lid.
Second, increasing the amount of the gel material 114 dispensed could have a negative impact on the function of some sensor devices in a strip or magazine. The precise amount of the gel material 114 dispensed, and the amount of capillary action that occurs, may vary from one sensor device to the next. Therefore, without additional gel material 114, the bond wires and dies of some sensor devices might be adequately covered. If the amount of the gel material 114 is increased for these adequately-covered sensor devices, then the height of the gel material 114 over the pressure-sensitive active region of the P-cell may become too great to allow for accurate sensing of the ambient atmospheric pressure.
Further adding to this height issue, in some implementations, a gel cap 130 is applied to the P-cell 106 before application of the gel material 114. Like the gel material 114, this optional gel cap 130 enables the pressure of the ambient atmosphere to reach the pressure-sensitive active region on the top side of the P-cell 106. When employed, gel caps similar to gel cap 130 are dispensed onto all of the P-cells on the strip or magazine, and the gel caps for all of the sensor devices on the strip or magazine are cured concurrently, before application of the gel material 114, by placing the whole strip or magazine into an oven.